(a) Field of the Invention
This invention relates, in general, to coated abrasive material and articles manufactured therefrom. More particularly, the invention relates to coated abrasive material comprising combinations of abrasive grain, in particular combinations of sol-gel process alumina abrasive grains with abrasive grains of cofused alumina-zirconia.
(b) Description of the Prior Art
Coated abrasive material typically comprises a relatively flexible backing member onto the front surface of which is coated an adhesive layer, the so-called "maker coat," followed by the application of a layer of abrasive grains onto the maker coat while such is still wet. The maker coat is then partially cured to a hardened layer after which another adhesive layer, the "size coat," is applied. The adhesive layers are then fully cured. The coated abrasive material is subsequently formed into various products, e.g., sandpaper, abrasive disks, endless belts, etc., depending somewhat upon the particular components of the coated abrasive material manufactured and its manner of processing.
The effectiveness of the material used as the abrasive grain in any particular coated abrasive material varies not only with the particular grinding conditions but also with the particular material or workpiece being ground. For example, certain abrasive grains have been found to be more effective for grinding stainless and low carbon steels at high metal removal rates than other abrasive grains. And certain other abrasive grains have been found more effective than others in relatively slow grinding applications. The explanation for the different effectiveness of a given abrasive material under differing grinding applications is often not clearly understood. Nevertheless, such appears, at least, to be related to, and dependent upon, the chemical composition of the abrasive material, and its microstructure, the related physical properties of the abrasive material such as hardness, fracture toughness, impact strength, and the thermal properties. The lack of such understanding naturally makes difficult the prediction of the effectiveness of a given abrasive material or grain for any particular grinding application, in the absence of prior experience. Thus, in determining whether or not an abrasive grain will be effective in any particular application, or the extent of such effectiveness, much depends upon trial and error.
Prior to the early 1960s, the abrasive grains used in coated abrasive material were, in general, either flint, emery, garnet, aluminum oxide, or silicon carbide, the latter two abrasives being man-made in electric furnaces according to well-known techniques. The so-called "fused alumina" provides, in general, a more chunky and tougher abrasive grain than silicon carbide and has been found over the years to be particularly well adapted to sanding operations on high tensile materials such as high carbon steel, alloy steels, tough bronze, and certain hard woods. When intended as a coated abrasive grain for woodworking, the crushing technique on the crude is varied to produce a somewhat sharper grain than that used generally for heavy metal working. Its hardness and sharpness has long made silicon carbide the ideal coated abrasive grain for sanding low-tensile metals, glass, plastics, fibrous woods, leather, enamel, and other relatively soft materials. Nevertheless, the applications for which such abrasive grains are now known to be best suited have been discovered only through much time and effort spent over the years, since these abrasive materials were first manufactured, in evaluating and testing of the abrasive grains in various applications and under various grinding conditions.
In the early 1960s, however, abrasive grains comprising a fused alloy-type alumina-zirconia (or "zirconia-alumina") composition became commercially available. Such abrasive materials are disclosed in U.S. Pat. Nos. 3,181,939; 3,891,408; and 3,893,826, all of which are assigned to Norton Company, the assignee of this application. The entire disclosures of these patents are incorporated herein by reference. As disclosed in the two later issued patents, the cofused alumina-zirconia abrasives disclosed therein were found quite effective in coated abrasive material for grinding 304 stainless steel and for grinding low carbon steels at high metal removal rates, but such abrasive material is disclosed to be less effective, as compared to fused alumina, at relatively slow grinding conditions. Nevertheless, since first introduced the alumina-zirconia abrasive grains have been discovered through empiricism to be a better across the board abrasive in coated abrasives in any use than fused alumina. Such more general use, however, comes at a premium compared to the cost of coated abrasive material containing only fused alumina abrasive grains.
More recently, there has become available another abrasive grain for use in the manufacture of coated abrasive material. Such novel abrasive material is obtained, in general, by drying and sintering a hydrated alumina gel which may also contain varying amounts of additives such as MgO and ZrO.sub.2. The dried material is typically crushed either before or after sintering to obtain irregular blocky shaped polycrystalline abrasive grain or grits in a desired size range. The grits may be later incorporated in abrasive products such as coated abrasive disks or belts.
Exemplary of prior art disclosing such abrasive material is U.S. Pat. No. 4,623,364, which issued on Nov. 18, 1986 to Norton Company, the assignee of the instant application. The entire disclosure of this patent is incorporated herein by reference. Other patents disclosing sol gel process (i.e., non-fused) alumina abrasive material and abrasive products containing such abrasive grains are U.S. Pat. Nos. 4,314,827; 4,543,107; 4,741,743; 4,744,802; and 4,800,685. U.S. Pat. Nos. 4,543,107 and 4,741,743 are both assigned to Norton Company, the entire disclosures of which are incorporated herein by reference.
U.S. Pat. No. 4,314,827 to Leitheiser et al. discloses abrasive grits made by the sol gel method in which the sintered abrasive grits are obtained from a calcium ion-free and alkali metal-ion free, dense aluminum oxide based abrasive material having a substantially microcrystalline structure of randomly oriented crystallites of alpha Al.sub.2 O.sub.3 and a modifying component with the alumina being a dominant continous phase. The mineral disclosed has a very fine crystal size throughout.
U.S. Pat. No. 4,543,107 discloses a vitrified (ceramic) bonded grinding wheel having sintered sol gel aluminous abrasive grits comprising sub-micron sized alpha alumina crystals. Contrary to prior art sintered gel type alumina or fused alumina, the use of the abrasive disclosed in this patent in vitrified wheels was discovered to be extremely sensitive to the firing temperature of the wheel. As a result, it was found necessary to control the viscosity of the bond and/or temperature of firing to avoid reaction between the bond and the abrasive.
U.S. Pat. No. 4,623,364 discloses a sol gel method for the manufacture of aluminous abrasive grits, and products other than abrasive grits such as coatings, thin films, fibers, rods or small shaped parts, having enhanced properties. In that patent it is disclosed that the conversion of the hydrated alumina to alpha alumina is facilitated by the introduction of seed material into the gel or the gel precursor prior to drying. To make abrasive grits the seeded gel is dried, crushed and fired. The abrasive grits so produced may be used in the manufacture of products such as coated abrasive discs and grinding weeels.
Sol gel abrasive material, and particularly seeded sol gel aluminous abrasives as disclosed in U.S. Pat. No. 4,623,364, have demonstrated substantial advantages over other abrasives, even such premium abrasives as alumina-zirconia, in certain coated abrasive applications since their introduction some few years ago. Compared to the aluminum-zirconia abrasive material, however, the sol-gel processed alumina abrasive has been discovered to be more selective in its performance in coated abrasive applications. Thus, it has been found, compared to alumina-zirconia abrasive, to provide superior performance in only certain metal grinding applications, e.g., in high stress areas such as the super alloys and aircraft alloys, e.g., the corrosion-resistant alloys of nickel and chromium such as "Inconel" and other high nickel metals, and in tool steels, etc. Nevertheless, in certain low carbon steel applications, its performance has been discovered to be only equivalent to, or perhaps slightly better than, the alumina-zirconia abrasive material. In general, an improvement in low carbon steel grinding applications using sol gel process alumina abrasive grains in coated abrasive material, compared to the alumina-zirconia grain, has been found only in high pressure, high stress, and high speed grinding applications. Such abrasive grain is considerably more costly than the alumina-zirconia abrasive and for that reason would not ordinarily be used in any such an application where no real benefit in performance is gained. In the grinding of stainless steel, titanium, and in some of the wood working applications, moreover, the sol gel process alumina abrasive has been discovered to be even less effective in coated abrasive material than the alumina-zirconia abrasive material.
Although the alumina-zirconia abrasive grains and those of sol gel process alumina have been found to provide superior performance in coated abrasive material in various broad applications, compared to the earlier used somewhat "inferior" abrasive minerals such as alumininum oxide, silicon carbide, etc., above-mentioned, the use of such in coated abrasive material comes at a premium in cost. Accordingly, there is continuing effort to satisfy the desire for and to provide coated abrasive material in which these superior abrasive grains are used, to take advantage of their superior performance characteristics but at a lower cost.
The combining of abrasive grains of two different abrasive minerals in coated abrasive material has long been done, for various reasons, e.g., to obtain a cheaper abrasive product for a particular application, to provide different or improved performance characteristics, improvement in cutting performance in a specific application, etc. In other cases, abrasive grains have been combined in coated abrasive manufacture to provide an improved manufacturing process or to reduce the overall cost of the coated abrasive material being manufactured. The combining of different abrasive grains in a coated abrasive product has been done by various manufacturing procedures, depending somewhat upon the specific performance or other results desired. In some cases, a mixture of abrasive grains of different chemical composition are provided in a single grain layer. At other times, the different abrasive grains are provided in separate grain layers, i.e., two maker coats and two grain coats are provided in the coated abrasive material. Other coated abrasive material may have one maker and two abrasive grain layers, with the relatively finer graded abrasive grains on the bottom and the coarser grains on top. Exemplary of prior art disclosing such coated abrasive material are U.S. Pat. Nos. 3,007,560; 3,606,764; 3,891,408; 3,893,826, 4,314,827; 4;409,791; 4,741;743; 4,744,802; and 4,800,685. Generally, however, where there is a combining of abrasive grains of different composition to lower the cost of the coated abrasive material there is a trade-off of some lesser performance. This is particularly so in the case where the layer of abrasive grains contacting the workpiece, i.e., the top layer in a dual layer of abrasive grains, is a combination of abrasive grains of different chemical composition.
U.S. Pat. No. 3,007,560, which isssued to Norton Company, discloses abrasive agglomerates for use in coated abrasive material which comprises a matrix of cellular glass and abrasive grit particles encapsulated in the cell walls. The abrasive grits can be co-fused alumina-zirconia, silicon carbide or alumina abrasive grits, or a mixture thereof. Generally, however, the finish obtained with such agglomerates is not as good as that obtained from coated abrasive material conventionally UP electrostatically coated with only either abrasive grain alone.
U.S. Pat. No. 3,606,764 discloses coated abrasive material comprising particles of sintered hard metal carbides, e.g., tungsten carbide, and particles of harder material such as alumina, zirconiaalumina, natural corundum, silicon carbide or emery bonded to a support member. Such coated abrasive material is preferably in the form of a disk or endless belt and is used particularly for abrading metal substrates, especially steel.
U.S. Pat. Nos. 3,891,408; and 3,893,826, both of which are assigned to Norton Company, assignee of the present application, each discloses coated abrasive material in which a double coat of abrasive grains are applied. Thus, there is disclosed in U.S. Pat. No. 3,891,408 coated abrasive material having a first layer of high purity alumina abrasive grains in first maker coat and a top layer of alumina-zirconia abrasive grains in a second maker coat. Such an abrasive product was discovered to outperform a double coated abrasive product containing only conventional high purity alumina abrasive grains in each of the layers, when evaluated on A-6 steel and 304 stainless steel. No blends of the alumina-zirconia abrasive grains with other abrasive grains in coated abrasive material, however, are disclosed in this patent.
In U.S. Pat. No. 3,893,826, the patentees therein disclose that one reason for the mixing of one abrasive grain with other abrasive grains of different shape and/or composition is to intensify the forces on that abrasive grain. Thus, to intensify the forces on certain alumina-zirconia abrasive grains having a lesser zirconia content, the patentees disclose a double coated product such as is disclosed in U.S. Pat. No. 3,891,408. Again, the double coated abrasive product containing a layer of fused alumina abrasive grains and a layer of alumina-zirconia grains was found to outperform coated abrasive material containing only fused alumina abrasive grains, in evaluations on 1018 high carbon steel and 304 stainless steel. No blends of alumina-zirconia abrasive grain with other abrasive grains in a single layer, however, are disclosed.
U.S. Pat. No. 4,314,827, earlier disclosed and which issued on Feb. 9, 1982, discloses, in Examples 44-46, blends of sol gel process alumina and fused alumina abrasive granules in varying proportions, used in nonwoven abrasive products, and evaluated on 1018 cold rolled steel. The test results show that the nonwoven abrasive products containing all or substantial amounts of the non-fused abrasive granules removed much more stock during the first 2-minute runs, and removed a greater amount of stock for the entire 8 runs, than did the abrasive products containing 50% or more fused alumina. Fused alumina, according to the patentees, outperforms alumina-zirconia in the same test. No blends of sol gel process alumina grains and alumina-zirconia grains are disclosed in, or even suggested, it is believed by, this patent.
U.S. Pat. No. 4,409,791 discloses coated abrasive material which comprises a dispersion of abrasive particles in a binding agent coated onto a flexible backing member. Suitable abrasives are flint, garnet, alumina or its mixtures with zirconia, silicon carbide, diamond, etc. The performance of such slurry coated abrasive mixtures is known to be somewhat less desirable than that obtainable from conventional UP coated abrasive grains due in part at least to the fact that abrasive grains of finer grit size are used.
U.S. Pat. No. 4,741,743, assigned to Norton Company, and which issued on May 3, 1988, discloses blends of sol gel process alumina abrasive grains and cofused alumina-zirconia abrasive grains used in the manufacture of grinding wheels. As disclosed in that patent, grinding wheels from such blends have performed better at intermediate grinding rates than wheels of either such abrasive material alone. Not only has this abrasive grain mixture found somewhat limited application in bonded abrasive wheels, the performance of abrasive grains in bonded abrasive applications have historically served as no predictor of what manner of performance may be achieved when such abrasive grains are used in coated abrasive material.
In U.S. Pat. No. 4,744,802, which issued on May 17, 1988, directed to a seeded sol gel process alumina that may be used in the manufacture of coated abrasive material, discloses that such abrasive grains are preferably blended with less expensive conventional abrasive grain such as fused alumininum oxide, silicon carbide, garnet, and fused alumina-zirconia. Nevertheless, even though the patentee discloses numerous examples, the patent does not, it is believed, disclose a single example of an abrasive grain blend, let alone one comprising the sol gel alumina disclosed and another abrasive.
U.S. Pat. No. 4,800,685, which issued on Jan. 31, 1989, discloses blends of sol gel alumina abrasive grain with conventional fused alumina abrasive grains and with silicon carbide grains. Such combinations, according to the patentee, have been found to provide superior performance in grinding wheels in the grinding of cast iron, a very specific application, compared to wheels containing either abrasive grain alone. There is no suggestion by this patent, however, that such an abrasive blend would be found suitable for use in coated abrasive material, or that such a blend could provide improved performance in coated abrasive material in any application. Neither does this patent disclose a blend of sol gel alumina abrasive grains and grains of alumina-zirconia. As a matter of fact, it appears from the disclosure that the abrasive blends disclosed in this patent are intended as a more economical replacement for grinding wheels comprising alumina-zirconia grains alone used in this application.
Norton Company, the assignee of the instant application, has for sometime marketed coated abrasive products in which the abrasive layer comprises a blend of fused alumina grains and cofused alumina-zirconia grains. In general such abrasive products do not provide as good a cut performance as coated abrasive products of 100% alumina-zirconia grains. Nevertheless, such an abrasive blend does result in coated abrasive material providing an upgrade in performance, compared to a product having only fused alumina grains. Somewhat more recently there has appeared in the marketplace a coated abrasive material in the form of conventional fibre discs that have a blend of sol gel alumina abrasive grain and fused alumina oxide grain in the abrasive layer. These coated abrasive discs have been found, in some cases at least, e.g., in the grinding of flat welds, to outperform coated abrasive material having only alumina-zirconia abrasive grains therein. The abrasive grain blends in these discs appear to be at least 50% by weight sol gel alumina abrasive grains. Such sol gel alumina blend coated abrasive products appear to be disclosed in U.S. Pat. Nos. 4,734,104; and 4,737,163. As disclosed in these patents a superior abrasive grain, e.g. one of zirconia-alumina or sol gel process alumina, is used to replace the coarse fraction, or at least a portion thereof, in a full grade of inferior grain such as fused alumina. The abrasive grain may be applied in two layers, if desired, as more particularly disclosed in the last mentioned patent, the coarse grain being applied in the second, or top layer.
In European Patent application No. 0 318 168, which was published on May 31, 1989, there is disclosed abrasive grains of sol gel process alumina having the shape of platelets. These abrasive grain platelets can be, according to the disclosure, blended with abrasive grains of other material, at least 15% by weight, and preferably 50 to 100% by weight, of the grains in the abrasive product being the platelet shape grains.